1. Field of the Invention
The invention proceeds from a method for temperature compensation of measuring signals of a fiber-optic sensor according to the preamble of patent claim 1.
2. Discussion of Background
In the preamble of patent claim 1, the invention refers to a prior art as is disclosed by K. Bohnert et al. in Coherence-Tuned Interrogation of a Remote Elliptical Core, Dual-Mode Fiber Strain Sensor, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 1, January 1995, pages 94-103. Described there are two fiber-optic measuring devices for measuring electric alternating voltages, in which the electric voltage is applied to a cylindrical quartz crystal. A resulting, periodic, piezoelectric deformation or change in circumference of the quartz crystal is transmitted to a two-mode glass fiber as sensor fiber. The periodic expansion of the sensor fiber caused thereby leads to a modulation of the phase difference between the two spatial, optical modes LP.sub.01 and LP.sub.11 (even) which are propagated in the sensor fiber. This phase modulation is proportional to the applied electric voltage. For the purpose of measurement, light is guided from a multimode laser via a single-mode fiber to the sensor fiber, and a further single-mode fiber and two modulators with a two-mode fiber as receiving fiber to 2 photodiodes which detect the interference pattern of the two modes. The differential, optical phase of the modes of the receiving fiber is controlled with the aid of an electronic control loop and two piezoelectric modulators in such a way that the phase modulation in the sensor fiber is precisely compensated again. The control voltage produced in the control loop and applied to the piezoelectric modulators is thus an image of the electric voltage to be measured. Instead of being operated in transmission, the sensor fiber can also be operated in reflection. An essential feature of the sensor consists in that disturbances do not impair the measuring signal through temperature fluctuations and mechanical vibrations acting on the connecting fibers between the transceiver unit and the sensor head. Changes in the length of the sensor and receiving fibers as a consequence of temperature changes likewise lead to optical phase shifts. However, as a rule, these are so slow that there is no problem in separating them from the periodic, electrically induced phase changes.
The piezoelectric effect in quartz is a function of temperature. Given a rise in temperature of the quartz, for example from 0.degree. C. to 100.degree. C., the sensor signal decreases by 2.15%. Without a correcting or compensating measure, this temperature dependence leads to a falsification of the measuring signal if the temperature of the sensor element is not held constant.